1. Field of the Invention
The present invention relates to a forming roll used for continuously manufacturing substrates for optical information recording mediums.
2. Related Background Art
Substrates for optical recording mediums have been hitherto prepared by processes such as the injection process, the 2P process, the casting process, and the compression process.
In these processes, however, substrates are formed sheet by sheet and hence a number of apparatus are required in order to prepare the substrates at high speed and in large quantities, thus making it difficult to prepare the substrates at a low cost.
On the other hand, processes of continuously preparing substrates used for optical recording mediums include, for example, a process in which a thermoplastic resin is made to have temperatures not lower than its thermal deformation temperature, and which is then extruded, passed between a pair of forming rolls, and formed into a resin sheet with uniform thickness, and at the same time, the preformatting information (e.g., tracking grooves and/or encoded information in the form of readable marks) on the roll is continuously transferred to the resin sheet. This process enables high-speed mass production of substrates. As methods of preparing a forming roll used in such a process, a method is known in which preformatting information is directly formed on a roll substrate, or a method in which a thin stamper is stuck on a roll substrate through an adhesive or the like. Of these methods, the method in which the stamper, having a preformatting information pattern, is stuck on the roll substrate can achieve easy production of the substrates for optical recording mediums. Even when the stamper has been broken, the stamper only may be replaced with new one, bringing about simplicity in the maintenance of the substrateforming roll.
In this method, however, the stamper used for forming must be bent after the shape of the roll surface when it is fixed on the roll, and hence the pattern on the surface of the stamper used for forming is stretched out, sometimes resulting in the transfer of a deformed pattern on the surface of the substrate. In particular, the pattern is stretched in the transport direction of the resin sheet, resulting in formation of inaccurate preformatting information on the substrates used for optical recording mediums. This has brought about the problem of failure of auto tracking particularly in the case of optical disks rotated at a high speed.